Sound recording circuit

ABSTRACT

Disclosed is a sound recording circuit capable of adjusting microphone sensitivity and preventing sound cracks caused by overly loud sound. The sound recording circuit includes: a microphone bias circuit configured to provide a bias voltage for a microphone circuit; an AC coupling capacitor configured to output an analog input signal according to a microphone signal of the microphone circuit; an analog amplifier circuit configured to output an analog output signal according to the analog input signal; an analog-to-digital converter configured to output a digital input signal according to the analog output signal; a digital amplifier circuit configured to output a digital output signal according to the digital input signal; and a signal detector configured to control an analog gain of the analog amplifier circuit, a digital gain of the digital amplifier circuit, and the bias voltage of the microphone bias circuit.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates a sound recording circuit, especially to asound recording circuit capable of preventing sound cracks caused byloud sound.

2. Description of Related Art

An anti-clamping recording function is widely used for a sound recordingdevice such as a cellphone, a camera, a video/audio recorder, a penrecorder, and a webcam. The function is mainly for preventing the audiosignals that are recorded to have sound cracks or clamping components,thereby reducing the distortions in the recording of sounds. The causeof sound cracks/clamping components in the recording often relates tothe design and/or the misuse of a sound recording device.

A general sound recording device having the function of dynamicaladjustment in microphone sensitivity and the function of anti-clampingrecording will usually include an analog amplifier, an analog-to-digitalconverter (ADC), a digital amplifier, and a signal detector. The analogamplifier amplifies a microphone alternating-current (AC) signal togenerate an analog amplified signal. The ADC generates a digital signalaccording to the analog amplified signal. The digital amplifiergenerates a digital amplified signal according to the digital signal andoutputs the digital amplified signal to a storage device for storage.The signal detector detects the output of the ADC to adjust the gain ofthe analog amplifier and/or the gain of the digital amplifier andthereby prevents the problem caused by audio clamping signal. However,although the above-mentioned sound recording device can deal with theclamping phenomenon of the analog amplified signal (that is caused bythe mismatch between the microphone sensitivity and the gain of theanalog amplifier) and the clamping phenomenon of the digital amplifiedsignal (that is caused by the mismatch between the microphonesensitivity and the gain of the digital amplifier) by the adjustment inthe gains of the analog amplifier and the digital amplifier, the soundrecording device is helpless when the microphone AC signal itself is aclamping signal; this circumstance is caused by the mismatch between thecharacteristic of a microphone and a microphone bias circuit of thesound recording device or caused by loud sound received by themicrophone. Due to the lack of techniques for dealing with themicrophone bias circuit, when the microphone AC signal itself is anaudio signal that has been clamped, the recording of sound willunavoidably include distortion regardless of the adjustment in the gainof the analog amplifier and/or the gain of the digital amplifier. Inaddition, the current techniques for adjusting microphone sensitivity indifferent recording modes or scenarios are still lacking.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a circuit being animprovement over the prior art.

An embodiment of the circuit of the present invention is capable ofbeing coupled to a microphone circuit and a storage circuit, andincludes a microphone bias circuit, an alternating-current (AC) couplingcapacitor, an analog amplifier circuit, an analog-to-digital converter(ADC), a digital amplifier circuit, and a signal detector. Themicrophone bias circuit is configured to provide a bias voltage for themicrophone circuit so as to allow the microphone circuit to generate amicrophone signal according to the bias voltage. The AC couplingcapacitor is configured to output an analog input signal according tothe microphone signal. The analog amplifier circuit is configured tooutput an analog output signal according to the analog input signal. Theanalog-to-digital converter is configured to output a digital inputsignal according to the analog output signal. The digital amplifiercircuit is configured to generate a digital output signal according tothe digital input signal and output the digital output signal to thestorage circuit. The signal detector is configured to control an analoggain of the analog amplifier circuit and a digital gain of the digitalamplifier circuit according to the digital input signal, and alsoconfigured to control the bias voltage of the microphone bias circuitaccording to the digital input signal. With the signal detectorcontrolling the bias voltage of the microphone bias circuit, the soundrecording circuit of this embodiment is capable of solving the problemof sound cracks caused by a microphone signal (i.e., clamping signalhere) exceeding the nominal maximum output limit of the microphonecircuit, and/or capable of executing the function of dynamicaladjustment in microphone sensitively.

Another embodiment of the circuit of the present invention is capable ofbeing coupled to a microphone circuit and a storage circuit, andincludes a microphone bias circuit, an AC coupling capacitor, an analogamplifier circuit, a first ADC, a digital amplifier circuit, a firstsignal detector, a second ADC, and a second signal detector. Themicrophone bias circuit is configured to provide a bias voltage for themicrophone circuit so as to allow the microphone circuit to generate amicrophone signal according to the bias voltage. The AC couplingcapacitor is configured to output an analog input signal according tothe microphone signal. The analog amplifier circuit is configured tooutput an analog output signal according to the analog input signal. Thefirst ADC is configured to output a first digital input signal accordingto the analog output signal. The digital amplifier circuit is configuredto generate a digital output signal according to the first digital inputsignal and output the digital output signal to the storage circuit. Thefirst signal detector is configured to control an analog gain of theanalog amplifier circuit and a digital gain of the digital amplifiercircuit according to the first digital input signal. The second ADC isconfigured to generate a second digital input signal according to theanalog input signal. The second signal detector is configured to controlthe bias voltage of the microphone bias circuit according to the seconddigital input signal. With the second ADC and the second signal detectorcontrolling the bias voltage of the microphone bias circuit, the soundrecording circuit of this embodiment is capable of dealing with amicrophone signal (i.e., clamping signal here) exceeding the nominalmaximum output limit of the microphone circuit.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiments that areillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the sound recording circuit of the presentinvention.

FIG. 2 shows an embodiment of the microphone bias circuit of FIG. 1.

FIG. 3 shows another embodiment of the microphone bias circuit of FIG.1.

FIG. 4 shows yet another embodiment of the microphone bias circuit ofFIG. 1.

FIG. 5 shows several scenarios of the sound recording circuit of FIG. 1generating clamping signals.

FIG. 6 shows an embodiment of the signal detector of FIG. 1.

FIG. 7 shows an equivalent circuit of the microphone circuit of FIG. 1.

FIG. 8 shows another embodiment of the sound recording circuit of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses a sound recording circuit capable ofcontrolling the bias voltage for a microphone circuit to prevent theproblem of sound cracks and/or capable of dynamically adjusting thesensitivity of the microphone circuit.

FIG. 1 shows an embodiment of the sound recording circuit of the presentinvention. The sound recording circuit 100 of FIG. 1 includes amicrophone bias circuit 110 (MIC Bias), an alternating-current (AC)coupling capacitor 120, an analog amplifier circuit 130, ananalog-to-digital converter (ADC) 140, a digital amplifier circuit 150,and a signal detector 160.

Please refer to FIG. 1. The microphone bias circuit 110 is configured toprovide a bias voltage V_(BIAS) for a microphone circuit 10 so as toallow the microphone circuit 10 to reflect sound waves (as shown by thedashed-line arrows of FIG. 1) according to the bias voltage V_(BIAS) andthereby generate a microphone signal S_(MIC). The microphone biascircuit 110 includes an adjustable resistor (e.g., the adjustableresistor 210 of FIG. 2), an adjustable voltage source (e.g., theadjustable voltage source 310 of FIG. 3), or both of the adjustableresistor and the adjustable voltage source (e.g., the adjustableresistor 210 and the adjustable voltage source 310 of FIG. 4) while thesound recording circuit 100 can optionally include the microphonecircuit 10 that is a known or self-developed circuit. The AC couplingcapacitor 120 is configured to output an analog input signal S_(AIN)according to the microphone signal S_(MIC). The analog amplifier circuit130 is configured to output an analog output signal S_(AOUT) accordingto the analog input signal S_(AIN) and the analog gain G_(A) of theanalog amplifier circuit 130; the analog amplifier circuit 130 can be aknown or self-developed circuit. The ADC 140 is configured to output adigital input signal S_(DIN) according to the analog output signalS_(AOUT); the ADC 140 can be a known or self-developed circuit. Thedigital amplifier circuit 150 is configured to generate a digital outputsignal S_(DOUT) according to the digital input signal S_(DIN) and thedigital gain G_(D) of the digital amplifier circuit 150, and then outputthe digital output signal S_(DOUT) to a storage circuit 12; each of thedigital amplifier circuit 150 and the storage circuit 12 can be a knownor self-developed circuit, and the storage circuit 12 is included in orindependent of the sound recording circuit 100. The signal detector 160is configured to control the analog gain G_(A) of the analog amplifiercircuit 130 and the digital gain G_(D) of the digital amplifier circuit150 according to the digital input signal S_(DIN), and also configuredto control the bias voltage V_(BIAS) of the microphone bias circuit 110according to the digital input signal S_(DIN). With the signal detector160 controlling the bias voltage V_(BIAS) of the microphone bias circuit110, the sound recording circuit 100 of this embodiment is capable ofrelieving the sound distortion (e.g., sound cracks) caused by amicrophone signal (i.e., clamping signal here) exceeding the nominalmaximum output limit of the microphone circuit 10, and/or capable ofadaptively adjusting the sensitivity of the microphone circuit 10 fordifferent sound recording settings.

FIG. 5 shows several scenarios of the sound recording circuit 100generating clamping signals. These scenarios include:

-   -   (1) Scenario 1: The mismatch between the sensitivity of the        microphone circuit 10 and the gains of the analog amplifier        circuit 130 and the digital amplifier circuit 150 leads to the        digital output signal D_(DOUT) being a clamping signal.    -   (2) Scenario 2: The mismatch between the sensitivity of the        microphone circuit 10 and the gain of the analog amplifier        circuit 130 leads to the analog output signal S_(AOUT) and the        derivative signals thereof being clamping signals.    -   (3) Scenario 3: The mismatch between the microphone circuit 10        and the microphone bias circuit 110 or overly loud sound leads        to the analog input signal S_(AIN) and the derivative signals        thereof being clamping signals.        When the signal detector 160 detects Scenario 1, the signal        detector 160 adjusts at least one of the analog gain G_(A) and        the digital gain G_(D) so as to prevent the digital output        signal S_(DOUT) from being a clamping signal. When the signal        detector 160 detects Scenario 2, the signal detector 160 adjusts        the analog gain G_(A) and optionally adjusts the digital gain        G_(D) so as to prevent the analog output signal S_(AOUT) and the        derivative signals thereof from being clamping signals. When the        signal detector 160 detects Scenario 3, which means that the        signal detector 160 finds that the analog input signal S_(AIN)        is a clamping signal, the signal detector 160 adjusts the bias        voltage V_(BIAS) of the microphone bias circuit 110 and        optionally adjusts at least one of the analog gain G_(A) and the        digital gain G_(D) so as to prevent the analog input signal        S_(AIN) and the derivative signals thereof from being clamping        signals.

In an exemplary implementation, when the signal detector 160 detectsScenario 3, the signal detector 160 determines whether the analog inputsignal S_(AIN) is a clamping signal in accordance with a pattern of thedigital input signal S_(DIN) (e.g., successive binary bits such as “1 11 1 1 . . . ” while the number of these bits can be determined accordingto the demand for implementation), the analog gain G_(A) of the analogamplifier circuit 130, and the nominal maximum output limit TH_(MIC) ofthe microphone circuit 10, and accordingly determines whether the biasvoltage V_(BIAS) of the microphone bias circuit 110 needs to beadjusted. More specifically, the signal detector 160 may carry out thefollowing steps to determine whether the analog input signal S_(AIN) isa clamping signal: obtaining the signal strength of the digital inputsignal S_(DIN) or the signal strength of the analog output signalS_(AOUT) (e.g., 1.2 Vrms (root-mean-square voltage)) according to thepattern of the digital input signal S_(DIN); dividing the signalstrength of the digital input signal S_(DIN) or the signal strength ofthe analog output signal S_(AOUT) by the analog gain G_(A) (e.g., 20dB=10 times) or offsetting the effect of the analog amplifier circuit130 to obtain the signal strength of the analog input signal S_(AIN)(e.g., 1.2 Vrms/10=0.12 Vrms); then comparing the signal strength of theanalog input signal S_(AIN) with the nominal maximum output limitTH_(MIC) of the microphone circuit 10 (e.g., 0.1 Vrms) to determine thatthe analog input signal S_(AIN) is a clamping signal if the signalstrength of the analog input signal S_(AIN) is higher than the nominalmaximum output limit TH_(MIC) (e.g., 0.12 Vrms>0.1 Vrms).

FIG. 6 shows an embodiment of the signal detector 160. The signaldetector 160 of FIG. 6 includes an energy calculator circuit 610, acomparator circuit 620, and a control circuit 630. The energy calculatorcircuit 610 is configured to calculate and output an energy valueP_(AIN) of the analog input signal S_(AIN) according to the digitalinput signal S_(DIN) and the analog gain G_(A) of the analog amplifiercircuit 130; for instance, the energy calculator circuit 610 executes alook-up table operation or predetermined calculation according to thepattern of the digital input signal S_(DIN) to obtain the signalstrength of the digital input signal S_(DIN) or the signal strength ofthe analog output signal S_(AOUT), and then the energy calculatorcircuit 610 divides the signal strength of the digital input signalS_(DIN) or the signal strength of the analog output signal S_(AOUT) bythe analog gain G_(A) or offsets the effect of the analog amplifiercircuit 130 to obtain the energy value P_(AIN) of the analog inputsignal S_(AIN). The comparator circuit 620 is configured to compare theenergy value P_(AIN) with the nominal maximum output limit TH_(MIC) ofthe microphone circuit 10 and thereby output a comparison value Comp;for instance, the comparator circuit 620 compares the energy valueP_(AIN) of the analog input signal S_(AIN) with the nominal maximumoutput limit TH_(MIC) of the microphone circuit 10 and therebydetermines that the analog input signal S_(AIN) is a clamping signal ifthe energy value P_(AIN) of the analog input signal S_(AIN) is higherthan the nominal maximum output limit TH_(MIC) of the microphone circuit10. The control circuit 630 is configured to adjust the bias voltageV_(BIAS) of the microphone bias circuit 110 and optionally adjust atleast one of the analog gain G_(A) and the digital gain G_(D) inresponse to the comparison value Comp indicating that the energy valueP_(AIN) is higher than the nominal maximum output limit TH_(MIC).

FIG. 7 shows an exemplary microphone circuit equivalent to themicrophone circuit 10. The microphone circuit 700 of FIG. 7 is anelectret condenser microphone (ECM) including an electret component 710and a common-source junction field effect transistor (JFET) 720. Theelectret component 710 is capable of outputting different gate voltagesVg to the gate of the JFET 720 in response to the variation of soundwaves (as shown by the dashed-line arrows of FIG. 5), and thus theoutput (i.e., the microphone signal S_(MIC)) of the drain of the JFET720 will vary with the change of the gate-to-source voltage Vgs of theJFET 720, in which the source voltage of the JFET 720 in FIG. 7 is aground voltage or a low supply voltage. In addition, since theconversion relation between the signal output (i.e., the microphonesignal S_(MIC)) of the drain of the JFET 720 and the signal input (i.e.,the voltage Vg) of the JFET 720 will vary with the DC bias voltage(which is affected by the current Id determined by the bias voltageV_(BIAS)), the output of the JFET 720 (i.e., S_(MIC)) can be adjustedwith the proper control over the bias voltage V_(BIAS) and thedistortion of the output of the JFET 720 can be relieved.

FIG. 8 shows another embodiment of the sound recording circuit of thepresent invention. The sound recording circuit 800 of FIG. 8 includes amicrophone bias circuit (MIC Bias), an AC coupling capacitor 820, ananalog amplifier circuit 830, a first ADC 840 (ADC1), a digitalamplifier circuit 850, a first signal detector 860, a second ADC 870(ADC2), and a second signal detector 880. The microphone bias circuit820 is configured to provide a bias voltage V_(BIAS) for a microphonecircuit 80 (which is included in or independent of the sound recordingcircuit 800) so as to allow the microphone circuit 80 to generate amicrophone signal S_(MIC) according to the bias voltage V_(BIAS). The ACcoupling capacitor 820 is configured to output an analog input signalS_(AIN) according to the microphone signal S_(MIC). The analog amplifiercircuit 830 is configured to output an analog output signal S_(AOUT)according to the analog input signal S_(AIN). The first ADC 840 isconfigured to output a first digital input signal S_(DIN1) according tothe analog output signal S_(AOUT). The digital amplifier circuit 850 isconfigured to generate a digital output signal S_(DOUT) according to thefirst digital input signal S_(DIN1) and output the digital output signalS_(DOUT) to a storage circuit 82 (which is included in or independent ofthe sound recording circuit 800). The first signal detector 860 isconfigured to control an analog gain of the analog amplifier circuit 830and a digital gain of the digital amplifier circuit 850 according to thefirst digital input signal S_(DIN1). The second ADC 870 is configured togenerate a second digital input signal S_(DIN2) according to the analoginput signal S_(AIN). The second signal detector 880 is configured tocontrol the bias voltage V_(BIAS) of the microphone bias circuit 810according to the second digital input signal S_(DIN2). In thisembodiment, the second ADC 870 generates the second digital input signalS_(DIN2) according to the analog input signal S_(AIN) instead of thefirst digital output signal S_(DIN1)/the analog output signal S_(AOUT)so that the second signal detector 880 can execute detection accordingto the second digital input signal S_(DIN2) early in comparison with thesignal detector 160 of FIG. 1 and control the bias voltage V_(BIAS) ofthe microphone bias circuit 810 promptly; in brief, the prompt detectionof this embodiment can shorten the duration of the recording (i.e., therecording data stored in the storage circuit 82) including sounddistortion.

In an exemplary implementation, the second signal detector 880determines whether the analog input signal S_(AIN) is a clamping signalaccording to a pattern of the second digital input signal S_(DIN2)(e.g., successive binary bits, each of which is bit “1” while the numberof these bits can be determined according to the demand forimplementation) and the nominal maximum output limit of the microphonecircuit 80. More specifically, the second signal detector 880 may carryout the following steps to determine whether the analog input signalS_(AIN) is a clamping signal: obtaining the signal strength of theanalog input signal S_(AIN) according to the pattern of the seconddigital input signal S_(DIN2); and then comparing the signal strength ofthe analog input signal S_(AIN) with the nominal maximum output limit ofthe microphone circuit 80 to determine that the analog input signalS_(AIN) is a clamping signal if the signal strength of the analog inputsignal S_(AIN) is higher than the nominal maximum output limit. Inaddition, an embodiment of the second signal detector 880 is similar tothe signal detector 160 of FIG. 6 and includes: an energy calculatorcircuit configured to calculate and output an energy value of the analoginput signal S_(AIN) according to the second digital input signalS_(DIN2); a comparator circuit configured to compare the energy valuewith the nominal maximum output limit of the microphone circuit 80 andthereby output a comparison value; and a control circuit configured toadjust the bias voltage V_(BIAS) of the microphone bias circuit 810 inresponse to the comparison value indicating that the energy value ishigher than the nominal maximum output limit of the microphone circuit80.

Since those of ordinary skill in the art can appreciate the detail andthe modification of the embodiment of FIG. 8 by referring to thedisclosure of the embodiments of FIGS. 1-7, which implies that thefeatures of the embodiments of FIGS. 1-7 can be applied to theembodiment of FIG. 8 in a reasonable way, repeated and redundantdescription is omitted here.

It should be noted that people of ordinary skill in the art canimplement the present invention by selectively using some or all of thefeatures of any embodiment in this specification or selectively usingsome or all of the features of multiple embodiments in thisspecification as long as such implementation is practicable, whichimplies that the present invention can be carried out flexibly.

To sum up, the sound recording circuit of the present invention cancontrol the bias voltage for a microphone circuit and thereby reduceclamping signals caused by circuit mismatch or overly loud sound;accordingly, the problem of sound distortion such as sound cracks can berelieved.

The aforementioned descriptions represent merely the preferredembodiments of the present invention, without any intention to limit thescope of the present invention thereto. Various equivalent changes,alterations, or modifications based on the claims of present inventionare all consequently viewed as being embraced by the scope of thepresent invention.

What is claimed is:
 1. A circuit capable of being coupled to amicrophone circuit and a storage circuit, comprising: a microphone biascircuit configured to provide a bias voltage for the microphone circuit;an alternating-current (AC) coupling capacitor configured to output ananalog input signal according to a microphone signal of the microphonecircuit; an analog amplifier circuit configured to output an analogoutput signal according to the analog input signal; an analog-to-digitalconverter configured to output a digital input signal according to theanalog output signal; a digital amplifier circuit configured to generatea digital output signal according to the digital input signal and outputthe digital output signal to the storage circuit; and a signal detectorconfigured to control an analog gain of the analog amplifier circuit, adigital gain of the digital amplifier circuit, and the bias voltage ofthe microphone bias circuit according to the digital input signal. 2.The circuit of claim 1, wherein when the signal detector determines thatthe analog input signal is a clamping signal, the signal detectoradjusts the bias voltage of the microphone bias circuit.
 3. The circuitof claim 2, wherein the signal detector determines whether the analoginput signal is the clamping signal according to a pattern of thedigital input signal, the analog gain of the analog amplifier circuit,and a nominal maximum output limit of the microphone circuit.
 4. Thecircuit of claim 1, wherein the signal detector controls at least one ofa voltage, a resistor, and a current of the microphone bias circuit soas to control the bias voltage of the microphone bias circuit.
 5. Thecircuit of claim 1, wherein the signal detector includes: an energycalculator circuit configured to calculate and output an energy value ofthe analog input signal according to the digital input signal and theanalog gain of the analog amplifier circuit; a comparator circuitconfigured to compare the energy value with a nominal maximum outputlimit of the microphone circuit and thereby output a comparison value;and a control circuit configured to adjust the bias voltage of themicrophone bias circuit in response to the comparison value indicatingthat the energy value is higher than the nominal maximum output limit ofthe microphone circuit.
 6. The circuit of claim 1, wherein themicrophone bias circuit is dynamically adjusted to control sensitivityof the microphone circuit.
 7. A circuit capable of being coupled to amicrophone circuit and a storage circuit, comprising: a microphone biascircuit configured to provide a bias voltage for the microphone circuit;an AC coupling capacitor configured to output an analog input signalaccording to a microphone signal of the microphone circuit; an analogamplifier circuit configured to output an analog output signal accordingto the analog input signal; a first analog-to-digital converterconfigured to output a first digital input signal according to theanalog output signal; a digital amplifier circuit configured to generatea digital output signal according to the first digital input signal andoutput the digital output signal to the storage circuit; a first signaldetector configured to control an analog gain of the analog amplifiercircuit and a digital gain of the digital amplifier circuit according tothe first digital input signal; a second analog-to-digital converterconfigured to generate a second digital input signal according to theanalog input signal; and a second signal detector configured to controlthe bias voltage of the microphone bias circuit according to the seconddigital input signal.
 8. The circuit of claim 7, wherein when the secondsignal detector determines that the analog input signal is a clampingsignal, the second signal detector adjusts the bias voltage of themicrophone bias circuit.
 9. The circuit of claim 8, wherein the secondsignal detector determines whether the analog input signal is theclamping signal according to a pattern of the second digital inputsignal and a nominal maximum output limit of the microphone limit. 10.The circuit of claim 7, wherein the second signal detector controls atleast one of a voltage, a resistor, and a current of the microphone biascircuit so as to control the bias voltage of the microphone biascircuit.
 11. The circuit of claim 7, wherein the second signal detectorincludes: an energy calculator circuit configured to calculate andoutput an energy value of the analog input signal according to thesecond digital input signal; a comparator circuit configured to comparethe energy value with a nominal maximum output limit of the microphonecircuit and thereby output a comparison value; and a control circuitconfigured to adjust the bias voltage of the microphone bias circuit inresponse to the comparison value indicating that the energy value ishigher than the nominal maximum output limit of the microphone circuit.